A conventional electrophotographic image forming apparatus employs a process cartridge scheme in which a cartridge integrally includes a photosensitive body, charging means, developing means, cleaning means, and toner storage section and is detachably mounted in the image forming apparatus main body.
This cartridge scheme makes a further improvement in operability, and allows a user to facilitate the maintenance of the process means. This cartridge scheme is very popular in image forming apparatus main bodies.
The process means are classified into long- and short-life process means and housed in cartridges of long- and short-life process means. In this manner, the cartridges are prepared depending on the service lives of the main process means.
Examples are a developing cartridge in which the toner storage section and developing means are integrated, a drum cartridge in which an electrophotographic photosensitive body, charging means, and cleaning means are integrated, and the like.
Demand has recently arisen for a color electrophotographic image forming apparatus capable of forming color images. An introduction of a color image forming apparatus, which satisfies the following seven items, is expected:    (a) low running cost    (b) small space    (c) low energy    (d) high image quality    (e) high speed    (f) improvement of usability    (g) ecology
A conventional process cartridge or developing cartridge must be replaced with a new one when toner stored in the cartridge runs out. Most of the cartridges are recycled and reused by recycle systems of cartridge manufacturers or general recycling companies, but are finally processed as the waste.
Cartridges preferably have longer service lives to reduce the total amount of waste from the viewpoints of environmental protection and energy savings. The process means (e.g., an electrophotographic photosensitive drum and developing roller) and toner must have longer service lives.
Assume that the service life of the process means becomes long and that toner corresponding to this service life is stored. The total toner weight is proportional to the service life. For example, when the service life of the process means lasts 50,000 images, the necessary toner weight is 1.25 kg to 1.5 kg. The large amount of toner is integrally stored in a cartridge, the total weight and volume of the cartridge become large, and operability may degrade.
The image forming apparatus main body requires a frame structure for supporting such a heavy cartridge with high precision, resulting in high cost of the whole apparatus.
A conventional toner replenishing 2-component developing system has a hopper portion which stores toner in the image forming apparatus main body. Toner is fed in an order of a toner replenishing container, hopper portion, and developing unit. Even if the toner replenishing container becomes empty, toner in the hopper can be used, thus providing a certain margin for a replacement timing of the toner replenishing container.
The presence of the hopper portion mechanism increases the number of components and the size of the cartridge, thus degrading the operability and increasing the cost as described above. The certain margin for the toner replacement timing of the toner replenishing container makes it impossible to accurately detect the toner replacement timing and the amount of toner left in the toner replenishing container. Trouble may occur in the image forming process near the end of the service life when the toner almost runs out. This may typically appear in the formation of a color image.
As described above, unless the remaining toner in the toner replenishing container can be accurately grasped, a clear color image cannot be formed although unused toner is still left in the toner replenishing container. For this reason, the cartridge replacement timing may be set earlier, and accordingly the resources cannot be efficiently used for the longer service life described above.
Demand has further arisen for higher image quality in a color image forming apparatus. A toner replenishing amount must be more accurately controlled to make constant a ratio (toner density) of the toner of the developing unit and the carrier in the toner replenishing 2-component developing system.